This invention relates to medical diagnostic equipment and methods and is particularly concerned with hollow viscus tonometry and direct or remote sensing of pressure within organs such as intra-abdominal pressure (IAP) and related properties indicative of the condition of the internal organs.
Until the advent of the tonometric method (see U.S. Pat. No. 4,643,192, issued Feb. 17, 1987) few considered any aspect of acid-base balance when attempting to monitor or maintain the adequacy of tissue oxygenation. Yet acid-base balance is primarily determined by the balance between the protons released during the release of energy by ATP hydrolysis and the resynthesis of ATP by oxidative phosphorylation. The hydrolysis of ATP generates 150,000 mmols of H+ each day in a resting 70 Kg man. All, but the 1% of this fixed acid load excreted by the kidneys each day, is presumed to be consumed in the resynthesis of ATP by oxidative phosphorylation. When the delivery of oxygen fails to satisfy the energy needs of the tissue the rate of ATP hydrolysis exceeds the rate of synthesis and the pH falls as the degree of unreversed ATP hydrolysis increases.
It is now widely accepted that global measurements of oxygen delivery, consumption and extraction do not provide reliable information about the adequacy of local or even "global" tissue oxygenation in patients. The indirect measurement of gastric intramucosal pH (pHi) as described in U.S. Pat. Nos. 4,643,192; 5,158,083; 5,186,172 provides clinicians with a minimally invasive yet sensitive means of detecting the development of a tissue acidosis, and hence inadequacy of tissue oxygenation, in a region of the body that is one of the first to exhibit an inadequacy of tissue oxygenation in shock. Use of the measurement has revealed that some 50% to 60% of patients having major surgery and 80% of ICU patients develop an intramucosal acidosis during their illness despite the conventional appearance of being adequately resuscitated.
The degree and duration of the presence of a gastric intramucosal acidosis are highly sensitive measures of the risk of developing ischemic gut mucosal injury and its putative consequences, namely the translocation of bacteria and their toxins, cytokine release, organ dysfunctions and failures, and death from the organ failures. By providing an index of the adequacy of tissue oxygenation in one of the first parts of the body to exhibit dysoxia in shock the measurement of gastric intramucosal pH improves the opportunity to obtain advanced and accurate warning of impending complications and to intervene in time to prevent them. More importantly timely therapeutic measures that restore the intramucosal pH to normality and "gut-directed" therapies incorporating measures that reverse an intramucosal acidosis are associated with an improved outcome. "pH-directed" therapy has in addition been shown to improve outcome in a prospective randomized multicenter study of medical and surgical ICU patients.
The measurements of gastric intramucosal pH have revealed deficiencies in currently accepted practices. It has, for example, become apparent that empirical increases in global oxygen delivery may be redundant in some 40% to 50% of patients having major cardiovascular surgery who do not develop a gastric intramucosal acidosis and whose prognosis is excellent. It is further apparent that the vogue of increasing global oxygen delivery to supranormal levels cannot be relied upon to prevent or to reverse the presence of an intramucosal acidosis. Of particular concern is the intramucosal acidosis that may be induced by measures, notably the transfusion of red blood cells and dobutamine, that increase global oxygen delivery in patients who do not have an intramucosal acidosis but whose global oxygen delivery is considered too low.
Intra-abdominal pressure (IAP) measurement is increasingly used in clinical practice as a guide to intraperitoneal pathology and as a predictor of renal function. In particular, IAP monitoring may be useful in the increasing practice of minimally invasive procedures such as laparoscopic surgery. In laparoscopic surgery gas is insufflated into the peritoneum to provide an operating field and improve the field of vision. Steps are taken to monitor the IAP caused by the insufflated gas to reduce the chances of overinsufflation.
Increased IAP may be associated with a variety of clinical situations that may adversely affect cardiac, renal, respiratory, and metabolic functions. See Iberti, et al., The Determination Of Intra-Abdominal Pressure Using A Transurethral Bladder Catheter: Clinical Validation of the Technique Anesthesiology, Vol. 70, No. 1, January 1989, p. 47. Diebel, et al., Effect Of Increased Intra-Abdominal Pressure On Hepatic Arterial, Portal Venous And Hepatic Microcirculatory Blood Flow, The Journal Of Trauma, Vol. 33, No. 2, 1992, p. 279 and references cited therein which are expressly incorporated by references herein.
Examples of situations where IAP monitoring may be useful include trauma, aneurism repair, laparoscopic surgery, massive edema, and bowel distention. The measurement of IAP may be complimentary to monitoring intramucosal pH as studies have demonstrated an increase in IAP may cause a reduction of blood flow to the microcirculation of the gut thus causing a fall in intramucosal pH.
A number of different techniques have been used to measure IAP. The measurement of IAP was performed in the last century using rectal and oesophageal sounds connected to a Marcy tambour. Wagoner GW, American Journal of Medical Science 1926; 171: 697-707. Intra-abdominal pressure has been measured transrectally in dogs--Thorington JM, Schmidt CP, A Study of Urinary Output and Blood Pressure Changes Resulting in Experimental Ascites, Am J Med Sci 1923; 165: 880-890. Also used has been a Miller Abbott tube in the rectum and a Hamilton manometer in the stomach to record pressure--Bradley SE, Bradley GP, The Effect on Increased Intra-abdominal Pressure on Renal Function in Man, J Clin Invest 1947; 26:1010-1022. IAP has also been measured through invasive techniques namely via catheters inserted transcutaneously into the peritoneal cavity Diebel, et al., Journal Of Trauma, 1992, p. 280. Direct cannulation of the peritoneal cavity and the injection of air have also been used--Emerson H., Intra-abdominal Pressures, Arch Int Med 1911;7:754--Overholt RH, Intraperitoneal Pressure, Arch Surg 1931;22:691-703.
IAP monitoring techniques have been developed for intravesical methods where the bladder is partially filled with saline and the fluid pressure in the bladder has been monitored as an indicium of IAP. The original technique has undergone some modification by Iberti, et al.--Iberti TJ, Lieber CE, Benjamin E., Determination of Intra-abdominal Pressure Using a Transurethral Bladder catheter, Clinical Validation of the Technique, Anesthesiology 1989;70:47-50. The intravesical technique has become an accepted standard for IAP monitoring. While reliable, it is cumbersome to perform and interferes with the estimation of the patient's urinary output readings. Because the urinary tract is essentially sealed by the intravesical technique of IAP monitoring, it is also of limited value for continuous IAP monitoring because urine output cannot be indefinitely obstructed.